1. Field of the Invention
The present invention relates to wireless network communications, and more specifically, to a multiple-frequency antenna structure for use in wireless local area network (WLAN) application.
2. Description of the Prior Art
The rapid development of the personal computer coupled with users' desires to transmit data between personal computers has resulted in the rapid expansion of local area networks. Today, the local area network has been widely implemented in many places such as in the home, public access areas, and the work place. However, the implementation of the local area network has been limited by its own nature. The most visible example of the limitation is the cabling. One solution to this problem is to provide personal computer with a wireless network interface card to enable the personal computer to establish a wireless data communication link. Using a wireless network interface card, a personal computer, such as a notebook computer, can provide wireless data transmission with other personal computers, or with a host computing device, such as a server connected to a conventional wired-line network.
The growth in wireless network interface cards, particularly in notebook computers, has made it desirable to enable personal computers to exchange data with other computing devices and has provided many conveniences to personal computer users. As a key component of a wireless network interface card, the antenna has received much attention and many improvements, especially in function and size. FIG. 1 shows a PCMCIA wireless network interface card 8 used in a notebook computer. The card can be used with a PCMCIA slot built in a notebook computer. As shown, the wireless network interface card 8 comprises a main body 23, and an extension portion 12. The main body 23 further comprises driving circuitries, connectors, etc. The extension portion 12 comprises a printed antenna 10 for transmitting and receiving wireless signals. Presently, the antennas being used widely in a wireless network interface card include the printed monopole antenna, chip antenna, inverted-F antenna, and helical antenna.
Among them, the printed monopole antenna is simple and inexpensive. As shown in FIG. 2, a printed monopole antenna 20 comprises a feed-line 21 a primary radiating element 22, a ground plane 24, and a dielectric material 25. The current on the printed monopole antenna is similar to current on a printed dipole antenna, so the electric field created will be the same. The difference is that the ground plane 24 of the printed monopole antenna 20 will create mirror current, so the total length of the printed monopole antenna 20 is only λg/4, which is half of the length of a printed dipole antenna. The improvement on the length of an antenna is significant in application for wireless network interface cards. The definition of the wavelength λg described above is
      λ    g    =            1                        ɛ          re                      *          c              f        0            
Wherein c is the speed of light, f0 is the center frequency of electromagnetic waves, and εre is the equivalent dielectric constant and is between the nominal dielectric constant (around 4.4) of circuit board and the dielectric constant (around 1) of air. For example, if the center frequency is 2.45 GHz and the dielectric constant is 4.4, the length of the printed monopole antenna will be 2.32 cm. Since the space in a wireless network interface card reserved for an antenna is limited, an antenna with such length will not fit properly into a card, therefore, some modification for the antenna is required. In the U.S. Pat. No. 6,008,774 “Printed Antenna Structure for Wireless Data Communications”, whose contents are incorporated herein by reference, modification for such antenna is disclosed. As shown in FIG. 3, the shape of a printed monopole antenna 30 has been changed in order to reduce the size thereof. The concept of U.S. Pat. No. 6,008,774 is to bend the primary radiating element 22 of FIG. 2 into the form of a V-shaped primary radiating element 32 as shown in FIG. 3. Although the overall length of the primary radiating element 32 of U.S. Pat. No. 6,008,774 is still λg/4, however, the space needed for furnishing this modified primary radiating element 32 is reduced. The antenna 30 shown in FIG. 3 also comprises a feed-line 31, the primary radiating element 32, a ground plane 34, and a dielectric material.